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1. PhD Physics student need help on career planning

Hi everyone - I'm hoping for a bit of impartial advice about career prospects for a couple of different options I'm considering. I graduated from a Masters degree in Physics back in 2009 and since then have been mostly travelling and working, albeit in an unrelated industry (medical...
2. Polymer random walk question

I understand the mean square end-to-end distance of a polymer of stepsize b and number of units N is given by \left\langle R^{2}\right\rangle = \sum_{i,j=1}^{N} \left\langle r_{i} r_{j}\right\rangle + \left\langle \sum_{i \neq j}^{N} r_{i} r_{j}\right\rangle And that the cross terms drop...
3. Probability distribution function / propagator

I have the following equation: \frac{1}{2N^{2}}\int_{s} \int_{p} \left\langle (\textbf{R}_{s} - \textbf{R}_{p} )^{2} \right\rangle which describes the radius of gyration of a polymer. (the term being integrated is the average position between beads p and s) This is shown to be...
4. Kinetic theory question

The molecular flux (number of collisions per unit area per unit time) in Kinetic theory is given by F=\frac{1}{4} n \bar{c} where c bar is the average molecular speed and n is the density of molcules in the gas phase (molecules/m^3) I was wondering about the origin of the 1/4 term ...

6. Calculus question

Homework Statement I have the following expression: cos\theta = \frac{\vec{a}\cdot \vec{b}}{ab} (this is simply taken from a dot product rule for two vectors) However I need to find \nabla_{\vec{r}i} \theta Is there a way I can do it without involving differentials of arccos and...
7. Tricky differentiation

Hmm, I've transcribed the problem straight from the question sheet. I too had that thought. I'm not sure I follow you with your methodology though. If I try to solve the equation using the standard quotient rule, it gives me zero - is there a reason for that?
8. Tricky differentiation

Homework Statement Find \frac{dU_{ave}}{d\beta} where U_{ave}=\sum_{k}\left(\frac{U_{k}exp(-U_{k}\beta)}{exp(-U_{k}\beta)}\right) Homework Equations The Attempt at a Solution My answer is supposed to be -(U_{ave}^{2})+(U_{ave})^{2} However I keep getting zero. I can...
9. Probability / Energy question

Homework Statement - Initial conformational potential energy = 2500 J - NVT Metropolis Monte Carlo simulation, T = 300 K - New conformational potential energy = 5000 J What's the probability this will be accepted within the Metropolis scheme? Homework Equations Acceptance...
10. Dipole Moment interaction

LOL, ok probably talking to myself here. Still playing around with this, taken it further, although I'm pretty sure my final answer here is wrong... http://voy.trekcore.com/working.jpg [Broken]
11. Dipole Moment interaction

OK, I've got a little bit further (this seems brutal!) I've been able to see that the separations of the charges mentioned above are: q1 & -q2: \sqrt{(d+rcos\theta)^{2}+(rsin\theta)^{2}} -q1 & q2: \sqrt{(-d+rcos\theta)^{2}+(rsin\theta)^{2}} Simplifying: q1 & -q2...
12. Dipole Moment interaction

Homework Statement Consider two dipoles with moments u1 and u2 arranged as in the following diagram. Each dipole is depicted as two charges of equal magnitude separated by a distance d. The centre-to-centre separation of the two dipoles is the distance r. The line joining the two dipole...
13. Unit cells (crystal structure)

I'm trying to clarify for myself the difference between a Unit Cell, a Primitive Unit Cell and a Conventional Unit Cell. As far as I know, Primitive Unit Cells contain only one lattice point and are the smallest possible unit cell Unit Cells are the crystal's smallest building block, and...
14. Entropy at different temperatures

^ Well the E/T term in the entropy would automatically go to infinity at 0K...
15. Entropy at different temperatures

I'm having difficulty with this problem: Consider a two state system consisting of N distinguishable and indeppendent particles where each particle can occupy one of two states separated by an energy E. What is the entropy of the system at: (A) T=0 (B) T=infinity I'm assuming this...
16. Formula derivation, algebra

Dang, that will teach me to copy and paste! I'm sorry, Defennder, here's the correct expressions: \frac{V_{2}}{V}=\frac{p-p_{1}}{p_{2}-p_{1}}
17. Formula derivation, algebra

I'm trying to derive a formula but can't seem to work the algebra. I need to combine these two: V_{1}p_{1} + V_{2}p_{2} = N V_{1} + V_{2} = V to get this: \frac{V_{1}}{V} = \frac{p-p_{1}}{p_{2}-p_{1}} where p = N/V If anyone could show me the steps that would be a huge help...
18. Bravais lattices

Homework Statement A crystal has a basis of one atom per lattice point and a set of primitive translation vectors of a = 3i, b = 3j, c = 1.5(i+j+k) where i,j,k are unit vectors in the x,y,z directions of a Cartesian coordinate system. What is the Bravais lattice type of this crystal...
19. Damped Driven Harmonic Oscillator

Homework Statement I'm trying to follow through a derivation involving the equation of motion for the displacement x(t) of a damped driven harmonic oscillator. m\frac{d^{2}x}{dt^{2}}+\gamma x + \beta \frac{dx}{dt}=F_{0}cos(\omega t) Where cos(\omega t) = \frac{1}{2}\left( e^{i \omega...
20. Molecular Simulation

Hey all, I'm looking for some suggestions on topics concerning Molecular Simulation which I can research. I've been tasked with giving a talk and writing a report on one specific topic of this field but I'm unsure what to specialise in. Any and all advise and suggestions would be appreciated!!
21. 1 dimensional charge distribution

Homework Statement (Lead in to question: "Assume that charges +q, -3q, 2q lie at positions -2a, 0, +2a along the x-axis respectively.") I've calculated dipole/quadrupole moments about the origin as well as the exact potential at x=+10a, however I'm confused by this next part to the...
22. Integration change of variables

Yep, that works good, thanks for your help nicksauce!
23. Integration change of variables

^ I can, I was just seeking assurance that I'd calculated that L^3 prefactor correctly after substitution...
24. Integration change of variables

hmm, if that was the case, wouldn't the equation take the form \Delta E_{3}^{(3)} = \frac{2}{L}\frac{V_0}{L^2}\frac{L^{3}}{(3\pi)^{3}}\int^{3\pi}_{0}\phi^{2}sin^{2}\phi d\phi (Note the L^3/... prefactor) ??
25. Integration change of variables

[SOLVED] Integration change of variables Homework Statement An electron is confined in an infinite one dimensional well where 0 < x < L with L = 2 x 10^-10m. Use lowest order perturbation theory to determine the shift in the third level due to the perturbation: V(x) =...
26. Perturbation Theory energy shift

Homework Statement I'm trying to calculate the energy shift given an electron in a 1D harmonic potential has a wavefunction \Psi_{0}(x) = \left(\frac{m\omega}{\pi\hbar}\right)^{1/4}exp\left(\frac{-m\omega x^{2}}{2\hbar}\right) The shift in E_{0} = \frac{\hbar\omega}{2} = 2eV due to...
27. Perturbation Theory transmission probability

I'm trying to bridge the gap between several expressions describing the insertion of a constant perturbation: a_{f}(t) = \frac{1}{i\hbar} V_{fi} \int^{t}_{0} e^{i(E_{f}-E_{i})t'/\hbar}dt' = \frac{1}{i\hbar}V_{fi}\frac{e^{i(E_{f}-E_{i})t/\hbar} - 1}{i(E_{f}-E_{i})/\hbar} so...
28. Dirac Equation 2!

Yep, another quick question on the Dirac Equation! I've become slightly more clued about the use of the DE now in illustrating the negative energy problem in relativistic QM as well as the existence of spin, however one thing is still puzzling me. I've read this excerpt in a text: I'm...
29. The Landau Gauge

Could anybody explain to me the difference between a Landau Gauge and Symmetric Gauge? I know the Landau Gauge is given by A = (0,Bx,0) producing a constant magnetic field in the z direction. I am *assuming* (process of elimination!) that A = ½B × r = (-yB/2,xB/2,0) is an example of a...
30. The Dirac Equation

^ Ah! Got it, i\gamma^{\mu}\partial_{\mu} = m from the original eq. Yes, I see now! Thanks so much, Dick!